The invention relates to a method and apparatus for producing fissionable deposites for reactor dosimetry and, more particularly, to a method and apparatus for producing fissionable deposits of ultralow mass on a substrate to serve as a dosimeter with a permanent record of high neutron fluence.
Ultralow-mass fissionable deposits have proved useful as fissioning sources for solid state track recorder fission rate measurements in high intensity neutron fields. These fission rate measurements are used to derive information for neutron dosimetry purposes.
A solid state track recorder placed adjacent to a fissionable deposit records tracks from the recoiling fission fragments which result from the fissions in the deposit. If the fissionable deposit is sufficiently thin, the effects of self-absorption can be ignored. The number of these tracks observed with an optical microscope after chemical etching of the solid state track recorder is proportional to the number of fissions that has occurred in the fissionable deposit, which is proportional to the fission rate per atom of the fissionable deposit and the fluence of the neutron field, given by the integration of the neutron flux over the period of exposure. Thus, the number of fission fragment tracks per square centimeter, i.e., the track density, in the solid state track recorder can be used to calculate the fission rate per unit area in the fissionable deposit.
For typical high neutron fluence applications, such as reactor core dosimetry or reactor component dosimetry, it has been found that a limitation is placed on using solid state track recorders due to the maximum track density that can be used, usually about 10.sup.6 tracks/cm.sup.2, without excessive track overlap. In order to avoid excessively high track densities, low-mass fissionable deposits can be used to reduce the number of fissions that will occur at a given neutron fluence.
For example, in dosimetry applications for light water reactor pressure vessel surveillance, .sup.235 U deposits with masses as low as 1.5.times.10.sup.-13 gram are required to produce a usable track density in a solid state track recorder. Similarly, low masses of other isotopes, such as .sup.237 Np, .sup.238 U and .sup.239 Pu, are required for dosimetry in light water reactor pressure vessel surveillance.
It has been found that the technical problems associated with the production of such low-mass deposits can be overcome by using radioisotopic spiking/electroplating techniques to characterize the masses of these ultralow-mass fissionable deposits. For example, ultralow-mass deposits can be produced by an electroplating technique using, e.g., .sup.237 U (7 day half-life) as an isotopic spike for .sup.235 U and .sup.238 U, .sup.239 Np (2.4 day half-life) as a spike for .sup.237 Np, and .sup.236 Pu (1.85 y half-life) as a spike for .sup.239 Pu. These electroplating procedures have inherent limitations, however, due to radioisotopic spike limitations, such as chemical impurities, etc. which lead to minimum masses of each isotope that can be produced and still give a meaningful signal above background in a neutron field.
Extension of deposit fabrication techniques to lower masses is described in co-owned and co-pending U.S. application Ser. No. 897,466, filed Aug. 18, 1986, entitled METHOD AND APPARATUS FOR PRODUCING ULTRALOW-MASS FISSIONABLE DEPOSITS FOR REACTOR NEUTRON DOSIMETRY BY RECOIL ION-IMPLANTATION. The recoil ion-implanation techniques described therein use an alpha emitting source which is a radioactive parent of the isotope of interest to implant recoil ions resulting from alpha decay into a suitable substrate. For example, an .sup.241 Am source in thin layer form is placed next to a substrate layer in a vacuum. Each alpha decay of .sup.241 Am results in a .sup.237 Np ion with enough recoil energy to be implanted in the substrate. As described therein, fission deposits with masses appropriate for high neutron fluence dosimetry are thus prepared.
In order to accurately determine the fission rate of a solid state track recorder neutron dosimeter produced according to the recoil ion-implanation techniques described in the referenced application, both the number of fissions that has occurred and the mass of the fissionable deposit must be determined to high accuracy. The former is simply determined from the number of fission tracks formed in the solid state track recorder. The latter requires careful and time-consuming mass calibration measurements on the fissionable deposit. Once calibrated, detailed records of the mass calibration must be maintained to ensure that the fission rate of the solid state track recorder neutron dosimeter can be calculated after exposure. These mass calibrations are usually done radiometrically, requiring accurate determinations of the uncertainties due to isotope half life, radiation branching ratios, detector efficiency, etc.
In light of the above, a method and apparatus are desired for producing ultralow-mass fissionable deposits which serve as a dosimeter with a permanent record of high neutron fluence.